An Electron Multiplying Charge Coupled Device (EMCCD) is a type of CCD device and is normally manufactured using standard CCD fabrication techniques. Examples of known EMCCDs are provided by E2V under the Trade Mark L3 Vision, and by Texas Instruments under the Trade Mark Impactron. Like CCDs, EMCCDs may be used in cameras, or other apparatus, to capture images, or other optical data, by means of the photoelectric effect.
FIG. 1 shows a schematic view of a conventional Frame Transfer EMCCD, generally indicated as 10. There are other types of conventional EMCCD and it will be understood that the invention described hereinafter is also applicable to these. The EMCCD 10 comprises an array 12 of photoelectric cells 14, or photodetectors. These are commonly referred to as pixels. The array 12 typically comprises a conventional CCD array. The array 12 is commonly a two dimensional array, the number of rows and columns varying from device to device. In some cases, the array may be one dimensional, i.e. may comprise a single row of cells 14. In some CCD types, the array 12 may include a first, or image, section 16 and a second, or storage, section 18. During use, photons impinge upon the cells 14 in the image area 16, each cell 14 generating one or more electrons, i.e. a quantity of electrical charge, (by means of photo-generated electron-hole pairs the photoelectric effect) depending on the level of incident radiation. Each cell 14, and more particularly the data derived therefrom, provides an element, or pixel, of each image, or other data set, captured by the EMCCD 10. The second section 18 of the array may be referred to as the storage area, the data, or charge, generated at the image area 16 being transferred or shifted to the storage area 18 after capture. The cells 14 of the storage area 18 are usually masked, typically with a deposited layer of aluminium, to prevent photons from impinging thereon.
For two dimensional arrays, a shift register 20, normally a serial shift register, comprising a plurality of cells or stages 23 is associated with the array 12, through which data is read from the array 12 during use. Typically data, in the form of electrons or charge, is transferred from the array 12 into the shift register 20 one row at a time. Normally, one end 22 of the shift register 20 is connected to a charge to voltage amplifier 24, the data in the register 20 being shifted serially out of the shift register 20, via end 22, and through the amplifier 24 producing output signal 29.
The cells 14 and register 20 are formed from any suitable semiconductor material and the shifting, or transfer, of electrons, or charge, from one cell 14, 23 to another is effected by control signals in the preferred form of clock signals. The clock signals normally take the form of voltage signals which, by appropriate manipulation of the electric fields in the cells 14, 23, cause the electrons to move, or transfer between cells 14, 23. Typically, the transfer of data, or electrons, from image area 16 of the array 12 to the storage area 18 is controlled by a combined clocking of a first set of clock signals (applied at inputs 50) and a second set of clock signals (applied at inputs 52). The transfer of data, or electrons, from the storage area 18 (when present) is controlled by the second set of clock signals 52 and the transfer of data, or electrons, from the register 20 is controlled by a third set of clock signals (applied at inputs 54). Typically, each set of clock signals comprises at least two (sometimes three or four) clock signals, each signal within a set usually being referred to as a phase, each respective set being applied, in use, to each respective cell or stage 14, 23.
The key distinguishing feature of an EMCCD in comparison with a conventional CCD is the provision of a structure 26 between the end 22 of the shift register 20 and the output amplifier 24. The structure normally comprises a solid state electron multiplier and is commonly referred to as the EM gain register or Charge Multiplication Register. The EM gain register 26 normally takes the form of a shift register, usually a serial shift register, that may be generally similar to shift register 20 and so comprises a plurality of stages or cells 27. The transfer of data, or electrons, through or from the EM gain register 26 is controlled by a fourth set of clock signals (applied at inputs 56).
The principle of operation of an EMCCD is now described with reference to a typical 3 phase CCD shift register structure. FIGS. 2A and 2B show a schematic representation of a portion of a shift register, generally indicated as 21, that is suitable for use as the EM gain register 26. A plurality of electrodes or terminals 28 are associated with the register 21 for applying control signals to the register 21. Adjacent terminals 28 supply different or respective control signals so that a potential difference is created between adjacent terminals 28. The potential difference between adjacent terminals 28 creates an electric field at the associated portion of the register 21 thereby causing electrons to move, or transfer, along the register 21 from one stage or cell to the next. This is known as charge transfer. By appropriate co-ordination of the control signals, electrons can be shifted along the register 21 to the output of the register 21 to create an output signal for supplying to the amplifier 24. Hence, the data obtained from the array 12 is shifted along the register 21.
In the present example, the control signals include three voltage clock signals, or phases, that may be designated as R1, R2 and R3 (alternatively there may be 2 or more phases and corresponding control signals). As for normal CCD shift registers (for example register 20), each phase is applied, via a respective terminal 28, to each stage or cell of the register 21 in a fixed sequence (e.g. R1, R2, R3). In order that the shift register 21 may serve as an EM gain register, an additional terminal 28A is provided for each stage or cell, the additional terminal 28A supplying a fixed reference signal (e.g. a d.c. signal). By way of example, in FIGS. 2A and 2B, electrode 28A is provided between the electrodes that supply the R1 and R2 phases. Electrode 28A is held, during use, at a fixed potential and the adjacent electrode 28B (which in this example applies the R2 phase) is clocked, i.e. supplies a clock signal for the R2 phase, as normal, except that a higher voltage amplitude is used than is necessary for charge transfer alone. For example, for normal charge transfer the clock amplitudes are typically 6 to 15V depending on actual CCD and its exact design, and those required for EM gain are typically 20V to 50V depending on actual CCD and its exact design. The arrangement of the fixed voltage, or d.c., electrode 28A and the clocked electrode 28B, and the relatively large voltage difference between them (hereafter referred to as the EM voltage), results in an intense electric field (hereafter referred to as the EM field) in the associated portion of the register 21 which is sufficiently high that the transferring electrons cause Impact Ionization. Impact Ionization generates new electrons, i.e. multiplication or EM gain, as illustrated in FIG. 2B (where, for reasons of clarity, only the multiplication transfer is shown since the skilled reader will be familiar with normal charge transfer in CCDs). The multiplication of electrons per transfer is relatively small, typically around ×1.01 to ×1.015. This may seem insignificant but when executed over a large number of transfers, a substantial EM gain is achieved. For example, with ×1.015 multiplication per transfer performed over, say, 591 transfers (which is typical but not limiting), a gain of 1.015 to the power of 591 (a multiplication factor of 6630) is achieved. Such gain levels signify that EMCCDs possess sensitivities every bit as good as, and better than, the very best ICCDs and Electron Bombardment CCDs (EB-CCDs).
Taking the example above, one electron fed into the EM gain register 26 should give 6630 electrons out, although it is noted that this is 6630 on average and in reality there will be a spread of outcomes. The statistical nature of this multiplication has been analysed and the general formula describing the probability, P(n,G,x), of a certain number of electrons resulting at the output of the EM gain register 26 for a given number of input electrons, and for a given EM gain, may be given by:
                              P                      (                          n              ,              G              ,              x                        )                          =                                            x                              (                                  n                  -                  1                                )                                      ⁢                          ⅇ                              -                                  x                  G                                                                                                        G                n                            ⁡                              (                                  n                  -                  1                                )                                      !                                              [        1        ]            
Where n is the number of input electrons, G is the total EM gain, and x is the number of output electrons.
The EM gain is a function of the EM voltage and of the operating temperature, the gain can typically be measured by taking the ratio of a given signal, e.g. a test signal, with the gain on and then with the gain off. Gain off can be achieved by using a lower clock amplitude at electrode 28B, i.e. an amplitude substantially the same or similar to that used for normal charge transfer in a CCD. The test signal is typically derived from a test image generated from a stable light source—no radiometric source or specific wavelength is required since only a ratio of the charge generated needs to be determined. Calibration of the EMCCD 10 at the time of manufacture, i.e. factory calibration, typically involves measuring the EM gain for different EM clock voltages and temperatures and then using this information to adjust the operating EM voltage range to give the desired performance. The dependence of EM gain on clock amplitude (or more specifically on the EM voltage) is very sharp (see FIG. 3, noting the log scale which shows typical EM gain as a function of EM clock voltage for a specific EMCCD) and so requires stable electronic design. The dependence on temperature is less sharp but good operating temperature stability is also required (see FIG. 4 which shows typical EM gain as a function of temperature for different EM clock voltages).
Some EMCCDs do have a particular problem, commonly known as the ageing effect, whereby the EM gain decreases with usage. The term usage relates to the cumulative amount of charge accelerated through the EM field. The ageing effect appears to be dependent on the amount of charge passed through the EM gain register 26 combined with the EM field strength that the charge is transferred through. It also appears to be very strongly dependent on the EM field strength and so, when the EMCCD 10 is operated at high EM gains, the ageing rate can be disproportionately greater. Fortunately it has been observed that the ageing effect itself decreases with time meaning that with proper use the device should remain useful for many years.
The explanation for the ageing effect is not fully understood but it is assumed that accelerating charge through high electric fields is causing charge to become embedded in an insulating layer (not shown), typically formed from silicon dioxide, that is provided between the EM electrodes 28B and the active silicon (or other semiconductor material) from which the stages 27 of the register 26 are formed. It is assumed that this slow build-up of charge effectively reduces the field strength produced by the electrodes 28B. Independent of the mechanism involved, this reduction in field strength can be compensated for by increasing the EM voltage. However, this normally requires a re-calibration of the EMCCD 10, essentially a repeat of the factory calibration. This calibration could be repeated and the adjustments to the EM voltage range made so as to bring the camera back to its original performance. Alternatively, the new calibration data could be recorded and used without any actual adjustments being made, but either way a re-calibration, or measurement of the EM gain is required.
The temperature dependence of the EM gain is not a significant problem because it does not change with time and is not sharply dependent on temperature. Therefore any re-calibration of the EM clock amplitude, or EM voltage, can be done at one temperature and extrapolated to other temperatures. The only proviso is that the temperature chosen should ideally be within the range of anticipated operating temperatures. High-end cameras would typically be operated at between −50° C. and −100° C., whereas volume market cameras would be operated typically between −20° C. and 0° C.
As mentioned above, the dependence of EM gain on clock amplitude is very sharp and because the ageing is strongly dependent on the EM field used inside the silicon (or other semiconductor material from which the register 26 is made), fields no higher than are required to achieve the desired gain should be used (i.e. within normal operating ranges), otherwise the ageing effect is accelerated. Therefore, any calibration procedure needs to be carefully controlled. This may be relatively straightforward at the time of manufacture, but is problematic when performed at the user's site, especially when being performed by an inexperienced user. Regular factory recalibrations are a solution, but this would necessitate the apparatus containing the EMCCD, typically a camera, being returned to the factory, and, in any event, the user cannot know that calibration is required without measuring the EM gain.
It would be desirable, therefore, to facilitate automatic re-calibration of EMCCDs without the need for skilled intervention by the user and/or extra external hardware (e.g. stabilised light source).